Materials that can selectively control light by efficient reflection, directed propagation or enhanced confinement have numerous applications as optical elements and devices. Photonic crystals (PCs) are a class of nanostructured materials with tunable reflection due to their periodic dielectric function, which creates a photonic bandgap where there are no allowed frequencies at which light can propagate through the material. One dimensional photonic crystals are already in widespread use, in the form of thin-film optics, with applications ranging from low and high reflection coatings on lenses and mirrors to color changing paints and inks. Higher dimensional photonic crystals are useful in non-linear devices and waveguides. Such devices rely on periodic nanoscale ordering of materials that affect the motion of photons trapped within the crystals.
Self-assembly is a powerful means of generating nanometer scale ordering in materials that often possess emergent photonic, plasmonic, magnetic, or other physical phenomena as a result of nanoscale structure control. However, self-assembled materials with resonant optical properties such as photonic band gaps are often difficult to achieve, as they require structures with large periodicities that are comparable to the wavelengths of light being manipulated.
Recently, brush block copolymers (BBCPs) have shown promise as building blocks for self-assembled photonic bad gaps in the UV, visible and IR. Brush block copolymers are large molecular weight structures that rapidly self-assemble into 1-D stacks with periodicities on the order of 100s of nanometers. These stacks are 1-D photonic crystals—they reflect light of a specific set of wavelengths that is determined by the thickness of the BBCP layers in a stack. In order to make these stacks reflect infrared light, they need to have layer thicknesses on the order of >250 nm. Such BBCPs can be readily synthesized by using a highly active ruthenium metathesis catalyst to polymerize norbornene-terminated polymer macromonomers via ring-opening metathesis polymerization, generating high molecular weight (MW, up to ˜6.5 MDa) polymers with relatively low dispersity (FIG. 1). Because of the steric hindrance that the macromolecule brushes impose upon the polynorbornene backbone, initial attempts were unable to produce structures that reflected these higher wavelengths, or if they did they were not very reflective and exhibited a significant amount of scattered light in the visible region of the spectrum. This is because the ordering of the BBCPs large enough to generate these stacks was very poor, due to the polydispersity in BBCP length and the rigidity of the BBCPs preventing the stacks from forming properly. In brief, the BBCPs act as rigid rods—if not all rods are the same length, it is difficult for them to properly pack without gaps in the assembled stacks.
Major challenges remain in the development of BBCP-based photonic bad gap materials, including synthesizing films that are reflective in the telecomm wavelength regime ˜1200-1650 nm) without being opaque in the visible, enhancing the processability of BBCPs during and after their synthesis, and incorporating different functional groups that would enable applications such as alignment, crosslinking, or manipulation of refractive indices. Although different macromonomers have been used to attempt to address some of these challenges, the difficulty inherent to generating such high MW polymers with low dispersity requires a re-optimization of the synthesis protocol for each new type of BBCP or macromolecule brush architecture.
The present invention is directed to solving at least some of these problems.